The powertrain in a conventional rear wheel drive vehicle includes a rear axle or differential system that may include axle members and gear sets, transmitting power from a drive shaft to the axle members to propel the vehicle. The rear axle gear sets may be lubricated by lubricating oil to ensure smooth operation of the rear axle differential. Viscosity and other fluid properties of the lubrication oil are a function of temperature and affect the efficiency and performance of the rear axle system and thus the vehicle. The rear axle lubricating oil may be less viscous with increasing temperature and may be more viscous with decreasing temperature. For example, at engine cold start the lubricating oil may be cold and hence, more viscous than desired. Conversely, for example, at high engine load the rear axle lubricating oil may be over-heated and may be less viscous than desired. To minimize friction loss and to reduce wear of the rear axle gears, which may result in reduced fuel efficiency, it is desirable to monitor the rear axle lubrication oil temperature and to maintain the lubrication oil temperature within a specified temperature range for optimal lubrication of the rear axle gears.
To address the problem of differential warm up, an example method is shown in U.S. Pat. No. 6,899,074, including the use of a coolant loop with a heat exchanger system coupled to the rear axle to regulate the temperature of the rear axle lubrication oil. In another approach shown in U.S. Pat. No. 8,485,932, an electric heating and cooling element is coupled to the rear axle to regulate the temperature of the rear axle lubrication oil.
However, the inventors herein have recognized potential issues with such systems, including the absence of a means to capture and store additional heat energy that may be available in a rear axle coolant system and the ability to deplete the stored heat energy to meet immediate rear axle lubrication oil heating demands. Accordingly, a system is provided herein to at least partly address the above issues. In one example, a rear axle coolant system may include an exhaust heat recovery system to transfer heat from an engine exhaust system to a cooling fluid, a rear-axle heat exchanger to transfer heat between the cooling fluid and rear axle lubrication oil, and a thermal battery system positioned intermediate the exhaust heat recovery system and the rear-axle heat exchanger, the thermal battery system configured to store excess heat from the exhaust heat recovery system. In one example, the rear axle coolant system may further include a controller storing instructions executable to, responsive to a first condition, bypass exhaust gas around the exhaust gas heat recovery system and flow cooling fluid from the thermal battery system to the rear-axle heat exchanger, and responsive to a second condition, bypass exhaust gas around the exhaust gas heat recovery system and flow cooling fluid from the exhaust gas heat recovery system to the rear-axle heat exchanger, bypassing the thermal battery system and flowing cooling fluid through the rear-axle heat exchanger to remove heat from the rear axle lubrication oil. In one example, the first condition may comprise the rear axle lubrication oil temperature above a threshold temperature and thermal battery system temperature lower than the rear axle lubrication temperature, and the second condition may comprise the rear axle lubrication oil temperature above the threshold temperature and thermal battery system temperature equal to or greater than the rear axle lubrication temperature.
In this way, the temperature of rear axle lubrication oil may be regulated through an exhaust gas heat recovery and storage system, including an exhaust gas heat exchanger and a thermal battery system coupled to a rear axle coolant loop communicating with a rear axle heat exchanger. The thermal battery system may store unused, additional heat recovered from the exhaust gas and may provide the stored thermal energy back to the rear axle coolant loop when there is a need for additional heat energy. The coolant flow through the above mentioned heat exchangers and the coolant loop may be regulated by a coolant pump and associated valves, controlled by a controller receiving sensor input from sensors located along the coolant loop.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.